Numerical simulations on the relative importance of starbursts and AGN in ultra-luminous infrared galaxies

Abstract

We investigate the relative importance of starbursts and AGN in nuclear activities of ultra-luminous infrared galaxies (ULIRGs) based on chemodynamical simulations combined with spectrophotometric synthesis codes. We numerically investigate both the gas accretion rates (m_acc) onto super massive black holes (SMBHs) and the star formation rates (m_sf) in ULIRGs formed by gas-rich galaxy mergers and thereby discuss what powers ULIRGs. Our principal results, which can be tested against observations, are as follows. (1) ULIRGs powered by AGN can be formed by major merging between luminous, gas-rich disk galaxies with prominent bulges containing SMBHs, owing to the efficient gas fuelling m_acc > 1 M_sun/yr of the SMBH. AGN in these ULIRGs can be surrounded by compact poststarburst stellar populations (e.g., A-type stars). (2) ULIRGs powered by starbursts with m_sf ~ 100 M_sun/yr can be formed by merging between gas-rich disk galaxies with small bulges having the bulge-to-disk-ratio (f_b) as small as 0.1. (3) The relative importance of starbursts and AGN can depend on physical properties of merger progenitor disks, such as f_b, gas mass fraction, and total masses. For example, more massive galaxy mergers are more likely to become AGN-dominated ULIRGs. (4) For most models, major mergers can become ULIRGs, powered either by starbursts or by AGN, only when the two bulges finally merge. Interacting disk galaxies can become ULIRGs with well separated two cores (> 20kpc) at their pericenter when they are very massive and have small bulges. (5) Irrespective of the choice of model, interacting/merging galaxies show the highest accretion rates onto the central SMBHs, and the resultant rapid growth of the SMBHs occur when their star formation rates are very high.